Recently, it is required that semiconductor devices have a high breakdown voltage and a low ON-state resistance. A SJ (super junction) structure type semiconductor device is well-known to have such characteristics. The SJ structure is formed in a drift region of the device so that an N conductive type column (N column) and a P conductive type column (P column) are alternately and periodically arranged in a direction that is perpendicular to a current flowing direction of the device. That is, if the device is a vertical MOSFET (Metal Oxide Semiconductor Field Effect Transistor) device, a plurality of N columns and P columns, which collectively form the SJ structure, are horizontally aligned to allow current to vertically flow. The SJ structure provides a low ON-state resistance by adjusting the impurity concentration of the current path to be relatively high and provides a high breakdown voltage by designing the SJ structure to be fully depleted during the OFF-state.
Generally, semiconductor devices have a chip configuration in which an active region is located at the center of the chip and a peripheral region surrounds the active region. In a case of the above mentioned vertical MOSFET device, a plurality of MOSFET cells are formed at the center of the chip as the active region.
In order to attain a high breakdown voltage, it is required for the SJ structure to be located not only in the cell region, the active region, but also in the peripheral region. When the SJ structure is continuously formed in both the active and peripheral regions, it is possible for the completely depleted region to expand to the peripheral region. Thus, the high OFF-state breakdown voltage is realized both in the active and peripheral regions.
In a conventional device, it may be preferred for the device to be designed so that the peripheral region has a breakdown voltage higher than that of the active region. This is because, when the device is used to drive an inductive load and an avalanche breakdown occurs in the peripheral region, an over-current due to the avalanche breakdown locally constricts the peripheral region, and the device may be destroyed. That is to say, it may be preferred that an avalanche breakdown should be made occur in the active region, which has a relatively large area. That way, the large energy of the avalanche breakdown will be consumed and absorbed in the active region.
For example, JP-A-2002-134748, which corresponds to U.S. Pat. No. 6,700,141, discloses a SJ type vertical MOSFET device that improves the avalanche withstanding capability under an inductive load.
The SJ type vertical MOSFET device according to the above publication has a SJ structure, which is formed in both the active and peripheral regions and is designed to have a higher breakdown voltage at the peripheral region than at the active region. The above publication shows an example in which a heavily doped intermediate drain layer is located between an undermost common drain layer and the SJ structure in the active region, and the vertical thickness of the SJ structure in the active region is reduced by the thickness of the intermediate drain layer. Making the thickness of the SJ structure thin in the active region can facilitate the priority occurrence of breakdown in the active region, which prevents a possible breakdown in the peripheral region. Furthermore, the above publication also shows another example in which, although the SJ structure has the same thickness in both the active and peripheral regions, the P and N columns in the active region have an impurity concentration distribution that is made relatively high near the undermost common drain layer. Making the high impurity concentration portion in the active region can restrain the expansion of the depleted region into the active region, which facilitates the priority occurrence of breakdown in the active region and thus prevents a possible breakdown in the peripheral region.
However, the above-described vertical MOSFET devices are academic and unrealistic because of difficulties in manufacturing. Generally, the SJ structure is formed by producing a high resistive layer on a heavily doped common drain layer, forming trenches that reach through the high resistive layer to the common drain layer, filling the trenches with opposite conductivity type epitaxial layers and thereby defining the P and N columns of the SJ structure using the trench-filling epitaxial layers and the partitioned high resistive layers, respectively.
In the examples proposed in the above publication it is necessary to differentiate the depths of the trenches in the active and peripheral regions or to differentiate the impurity concentrations of the P and N columns in the active and peripheral regions. These differentiating designs in the SJ structure complicate the manufacturing processes and require many steps.